Andrea Francke, Michael Hoffmann. The Euclidean degree-4 minimum spanning tree problem is NP-hard. In John Hershberger, Efi Fogel, editors, Proceedings of the 25th ACM Symposium on Computational Geometry, Aarhus, Denmark, June 8-10, 2009. pages 179-188, ACM, 2009. [doi]
Abstract is missing.